The best way to write in Hindi/Marathi (Devnagari) on a Windows 2000 or newer systems is to install the Indic Support. Then learn to use the Devnagari Inscript Keyboard. It takes some time to learn but make it a point to type a few lines in Devnagari everyday and in a month or so you can get used to it. See Devnagari Keyboard Layouts on Microsoft's page. Select Hindi or Marathi Keyboard and it'll show you the layout. The Hindi and Marathi keyboards have slight differences. I prefer Marathi Keyboard because Marathi uses western style punctuation so that is default (like the period etc. instead of dandak etc in Hindi) and decimal numbers are in Devnagari by default. You can get international numbers with RightAlt- key. This behavior is exactly reverse in Hindi Traditional keyboard.
You can also use Windows On-Screen Keyboard, Start -> Programs -> Accessories -> Accessibility -> On-Screen Keyboard.
